Magnetically assisted abrasive cleaning and polishing (hereinafter called `magnetoabrasive machining` for brevity) is most efficient when applied to machining intricately shaped workpieces produced by die-forging, investment-casting, press-forming, or rolling, whereby the shape and size of a blank are maximally approximated to those of a finished product so that the latter needs only improvement in the surface roughness of the machined surface.
One prior-art device for magnetoabrasive machining of workpieces (cf., e.g., USSR Inventor's Certificate No. 315,577, Cl.B24b 31/10, 1971) is known to feature its magnetic system arranged on the same side as the workpiece surface being machined, with a clearance therebetween filled by a ferroabrasive dust. The magnetic system of the device consists of a cylinder whose end face carriers two coaxial ring-shaped unlike-polariry poles, and a magnetic coil fitted onto the inner pole. When the coil is energized a magnetic field is induced in the working clearance (magnetic air gap) which attracts the grains of the ferroabrasive dust to the ring-shaped poles. The majority of the abrasive grains become oriented across the ring-shaped poles to form a bridge therebetween, while but a minor part of the grains form a brushlike structure on the end face of the inner ring-shaped pole under which the coil turns are located. The aforesaid brush is arranged normally to the surface of the workpiece being machined. With the device receiving rotary motion and the workpiece reciprocating, the brush formed on the end face of the inner ring-shaped pole, polishes the workpiece surface.
However, workpiece machining with the afore-discussed device is unproductive and low-efficient since the value of magnetic induction in the machining zone is low and the force of pressing the ferroabrasive grains against the surface being machined is inadequate for the cutting process to occur. Moreover, some of the trains formed by the abrasive grains oriented lengthwise of the magnetic lines of force rotate together with the poles, while other grains remain on the surface on the workpiece being machined, thus impeding the machining process, affecting its productivity and causing premature wear of abrasive grains. All of this results in a badly deteriorated machining process.
In addition, the device mentioned above requires electric power to be supplied from an external source to establish a magnetic field, features a great mass and large-sized magnetic coils, which impairs the access to the working zone of the device, especially when machining small workpieces. Thus, the device will do only for machining large-sized workpieces.
To some extent the aforementioned disadvantages are eliminated in a device for magnetoabrasive machining of workpieces (cf., e.g., a USSR Inventor's Certificate No. 674,874, Cl.B24b 31/10, 1979), which comprises a carrier of a nonmagnetic material provided with a fixture for being held to the shaft of a rotary mechanism, and at least one permanent magnet located in a respective magnetic circuit fixed on a carrier.
The device considered above incorporates also another carrier positioned coaxially with the former one and carrying permanent magnets equal in number to those mounted on the former carrier.
One of the carriers carrying the magnetic circuit is fixed stationary, while the other carrier is turnable with respect to the former carrier. When in working position the permanent magnets of the carriers face one another with their unlike-polarity poles. When in nonworking position used for removal of the ferroabrasive dust from the surface of the working poles, the nonworking poles of the carrier-mounted permanent magnets are offset with respect to one another.
To effect machining the device in question is positioned with some clearance to the surface of the workpiece being machined and the clearance is filled with ferroabrasive dust. Then the device and workpiece are set in rotation and reciprocating motion, respectively. As a result, the ferroabrasive grains are entrained by the forces of the magnetic field established across the working poles of the device, to polish the surface of the workpiece being machined.
The magnets of unlike polarity in each of the carriers of the device are spaced apart from one another a distance equal to the diameter of the magnet, while the magnets of the stationary carrier are distant from the working gap for a length equal to the thickness of the movable carrier. Such an arrangement of the magnets extends the magnetic circuit, increases its resistance and magnetic dispersion flux, and reduces magnetic flux across the working gap. This means that normal forces Fmy pressing the grains of ferroabrasive dust against the magnet poles, and horizontal forces made up by radial components Fmx.sub.r and tangential components Fmx.sub..tau., are diminished. Once forces Fmy have been reduced part of the ferroabrasive grains slip with respect to the magnet poles, whereby the grains polish the surface of the poles rather than that of the workpiece being machined, which reduces the effective life of the ferroabrasive grains. When forces Fmx.sub.r are reduced this increases the amount of ferroabrasive grains expelled from the working gap by centrifugal forces, while reduction of the magnitude of forces Fmx.sub..tau. acting in the same direction as the cutting speed vector results in that ferroabrasive dust follows but unreliably the poles of the rotating carrier. As a consequence, ferroabrasive dust lags behind the rotating carrier with the poles and is displaced along the pole working surface in a direction opposite to the cutting speed vector, the working gap gets rid of ferroabrasive dust and the surface of the magnet pole becomes bare. Moreover, reduction of the magnitudes of forces Fmy, Fmx.sub.r and Fmx.sub..tau. effective in the working gap brings about reduction of the respective normal and tangential cutting force components. This, in turn, leads to badly affected productivity of the cutting process and its rapid damping.